[Haskell-cafe] Defining instance needs allow-undecidable-instance?
dagda at xtra.co.nz
Mon Mar 27 21:25:51 EST 2006
On Tuesday 28 March 2006 11:12, Daniel McAllansmith wrote:
> Hi, folks.
> I've got a working class and instances thereof. I would now like to change
> the class such that error behaviour is specified by MonadError, for the
> moment throwing String errors. When I try to add MonadError into the types
> I eventually hit the requirement for allow-undecidable-instances.
> Is there some way I can I avoid having to use this extension?
I've received a link off-list to a patch to GHC changing this behaviour, so
it looks like in the future I'll be able to remove the
allow-undecidable-instances flag and rely on just glasgow-exts.
> instance (Num i, Bounded i, Monad m, MonadError String m)
> => MonadSource i (SourceT i m) where
That aside, is what I have done, including a type in the context, considered
an acceptable solution for this sort of problem?
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